1. Field of the Invention
The present invention relates to a thin film transistor and a method of making the same, and more particularly to a thin film transistor for a liquid crystal display device (LCD) and a method of making the same.
2. Description of the Prior Art
Thin film transistors are widely used as switching elements for LCDs.
FIG. 1 is a schematic sectional view of an example of conventional thin film transistor. As shown in FIG. 1, the thin film transistor comprises an insulating substrate 11 over which a metal layer for a gate electrode is deposited. At one side of the substrate 11, a gate electrode 12 is provided, which is formed by patterning the metal layer. Over the entire upper surface of the resulting structure are deposited a gate insulating film 13 and an amorphous silicon film 14 in this order. The amorphous silicon film 14 is patterned, so as to provide an amorphous silicon film pattern above the gate electrode 12. A transparent electrode 15 is disposed on the gate insulating film 13 such that it is spaced from the amorphous silicon film 14. On the gate insulating film 13, a drain electrode 16 is also disposed such that it is overlapped with one upper surface portion of the amorphous silicon film 14 and an upper surface portion of the transparent electrode 15 facing the one upper surface portion of amorphous silicon film 14. A source electrode 17 is also disposed on the gate insulating film 13. The source electrode 17 is overlapped with the other upper surface portion of the amorphous silicon film 14. Over the entire upper surface of the resulting structure is formed a non-active layer 18 as a passive layer for the thin film transistor. Thus, the structure shown in FIG. 1 is obtained.
For improving the characteristics and yield of thin film transistors with the above-mentioned structure, there have been also proposed other thin film transistors such as a thin film transistor with a double gate insulating film structure including two gate insulating films 23-1 and 23-2 as shown in FIG. 2 and a thin film transistor with a double gate electrode structure including two gate electrodes 32-1 and 32-2 as shown in FIG. 3. They use glass substrates or quartz substrates as their insulating substrates 21 and 31. For gate electrodes 22 and 32, metals such as Al, Ta, Ti and Nb or alloys thereof are used. Also, a transparent anodized film made of Al.sub.2 O.sub.5 or Ta.sub.2 O.sub.5 is used as one gate insulating film 23-1 of the double insulating film structure. As the other gate insulating film 3-2, a nitride film made of SiN or an oxide film made of SiO.sub.2 is used.
For fabricating the double gate insulating film structure of the thin film transistor shown in FIG. 2, first, over an insulating substrate 21 is deposited an anodizable metal layer having a thickness of about 3,000 .ANG.. The metal layer is then subjected to a patterning for forming a metal pattern 22. The metal pattern 22 with the thickness of 3,000 .ANG. is then partially subjected to an anodization, so that its thickness portion of 1,500 .ANG. is anodized to form a first gate insulating film 23-1 with a thickness of about 2,000 .ANG.. The unanodized portion of metal pattern 22 constitutes a gate electrode which has a thickness of about 1,500 .ANG.. After the formation of the first gate insulating film 23-1 by the anode oxidation, a second gate insulating film 23-2 made of a nitride film or oxide film is formed over the upper surface of the resulting structure. Thus, the double gate insulating film structure is obtained.
Where a transparent anodized film is formed as the gate insulating film by using the anodization technique, as mentioned above, and assuming that D.sub.0, D.sub.1, and D.sub.2 in FIG. 4A represent the initial thickness of the metal pattern 22 for gate electrode, the thickness of the portion of metal pattern to be changed into the gate insulating film after the anodization, and the thickness of the unanodized portion of metal pattern to be used as the gate electrode, respectively, the thickness-D.sub.1 portion of the metal pattern prior to the anodization is changed into the anodized film 23-1 after the anodization, as shown in FIG. 4B. Actually, the anodized film 23-1 is not identical to the thickness D.sub.1 of the metal pattern 22 prior to the anodization, due to the volume expansion occurring during the anodization. Taking the volume expansion coefficient .alpha. into consideration, the thickness of anodized film 23-1 is therefore .alpha.D.sub.1. Accordingly, the step resulting from the formation of the metal pattern, which has an initial height Si of D.sub.1 +D.sub.2 prior to the anodization, has an increased height after the anodization, that is, a final height Sf.sub.1 of D.sub.1 +.alpha.D.sub.2. The volume coefficient .alpha. upon the anodization is 1.5 in Al, 2.5 in Ta, and 2.5 in Nb.
As a result, where the double gate insulating film structure is formed using the anodization technique, the step height between the insulating substrate and the gate electrode is increased due to the volume expansion caused by the anodization. Such an increase in step height results in an increase in short circuit generation rate at crossing portions of gate electrode and source/drain electrodes.
On the other hand, the transparent anodized film which is used as the gate insulating film, for improving the characteristics and yield of thin film transistors is formed by a chemical reaction occurring by a current at an anode. Where a plurality of thin film transistors are fabricated on one substrate, anodized films therefor are simultaneously formed under a condition that a plurality of gate electrodes are connected with one another. As a result, if the line width of each gate electrode is very small or irregular due to the gate electrode design or line defects, as shown in FIG. 5A, current flowing through the gate electrodes during the anodization may concentrate on gate electrode portions where the line width is reduced, as shown in FIG. 5B. Moreover, a short circuit may occur among gate electrodes, due to a poor adhesion of gate electrodes to the insulating substrate. Where such a short circuit problem is taken into consideration, the formation of insulating films using the anodization technique encounters a problem of a limitation on line width and line thickness of gate electrodes.
FIG. 6 is a schematic sectional view of an active matrix LCD employing the thin film transistor of FIG. 2.
As shown in FIG. 6, the LCD comprises a lower plate 60, an upper plate 70 and a liquid crystal 80 sealably contained a chamber defined between the lower plate 60 and the upper plate 70.
For fabricating such a LCD, first, on a lower insulating substrate 61 is formed a thin film transistor with the structure of FIG. 2, so as to prepare the lower plate 60. Thereafter, a formation of the upper plate 70 spaced from the lower plate 60 is carried out. For preparing the upper plate 70, first, on a upper insulating substrate 71 is formed a black matrix 72 disposed at a lower surface portion of the upper insulating substrate 71, which corresponds to the thin film transistor. On the entire lower surface of the resulting upper plate structure, an upper common electrode 73 is formed. Thereafter, a liquid crystal 80 is injected into a space defined between the lower plate 60 and the upper plate 70.
The conventional LCD with the above-mentioned structure encounters a reduction in opening rate due to a bonding tolerance occurring upon bonding the lower and upper plates 60 and 70, since the upper plate 70 has the black matrix 72. Furthermore, it also encounters a problem of a degradation in thin film transistor characteristic, since light beams are radiated to the thin film transistor of the lower plate 60 because of a light reflection occurring at the black matrix 72 of upper plate 70.